This invention relates to a slip-controlled brake system for automotive vehicles with a driven axle and a non-driven one. The brake system has a braking pressure generator connected to an auxiliary pressure source. The wheel brakes of a front wheel and of a rear wheel are connected to the braking pressure generator by way of two separate pressure medium circuits. The brake system includes wheel sensors and electonic circuits for the detection of the rotational behavior of the wheels as well as of the vehicle's velocity and for the generation of braking pressure control signals. The brake system is equipped with a normally open inlet valve each in the pressure medium line leading from the braking pressure generator to the two associated wheel brakes and with a normally locking outlet valve which after switching-over into the open position connects the pressure medium line which leads to the two wheel brakes with a pressure compensation reservoir.
Brake systems of this type are known where the wheels are connected to a braking pressure generator by way of two separate pressure medium circuits. In these systems, the allocation of the wheels to the brake circuits may be diagonal or it is possible to connect the righ-hand wheels to one circuit, the left-hand wheels being connected to the other circuit.
Contrary to systems with three brake circuits or even with individual slip control at each wheel, in such simplified brake systems only a joint control of the braking pressure is possible at one front wheel and at the associated rear wheel--i.e., at the rear wheel connected to the same circuit. In many situations this does not lead to any disadvantage compared with systems with independent bracking pressure control at the individual wheels. In case of some road conditions and driving situations it is more advantageous to control the braking pressure in accordance with the rotational behaviour of the front wheel; in other cases it will be more advantageous to control in accordance with the rotational behavior of the rear wheel. Under certain conditions, such as on icy roads, the control should even permit one of the two wheels to lock.
Further, it is known to install pressure reducers in the pressure medium line leading to the rear wheel as, due to the dynamic axle load shift, a higher braking pressure often is required at the front axle. For the same reason it will often be advantageous for the control quantity for the rating of the braking pressure to be derived from the front wheel. It has also been suggested to effect a switching-over in accordance with certain criteria so that temporarily the rear wheel instead of the front wheel governs or determines the braking pressure.
In some cases, however, the joint control of the braking pressure of the two wheels connected to one pressure medium circuit will be disadvantageous as compared with systems featuring individual control. It is thus an object of this invention to overcome this disadvantage and to provide a brake system which is limited to two pressure medium circuits with diagonal or right-and-left braking force allocation and in which, nevertheless, the pressure variation at the front and rear wheels of one pressure medium circuit is decoupled as far as possible in order to enable the slip control to be adapted to the varying road and driving conditions. Further, for the purpose of controlling the traction slip it should be possible to supply braking pressure to the driven wheels, only.